Cambridge IGCSE Chemistry Chapter 19 Organic Chemistry Study Notes and Assessment Quiz

Cambridge IGCSE Organic Chemistry Overview and Naming Conventions

• Definition of Organic Chemistry: Organic chemistry is the branch of chemistry focused on carbon-containing compounds, excluding simple oxides such as carbon monoxide ($CO$) and carbon dioxide ($CO_2$), as well as carbonates.
• Homologous Series: A family of organic compounds that exhibit the following defining characteristics:
  • They share the same general formula.
  • They possess the same functional group.
  • They undergo similar chemical reactions.
  • They show a gradual trend in physical properties (such as boiling points, melting points, and viscosity).
  • Successive members differ from one another by a $-CH_2-$ structural unit, corresponding to a difference in relative molecular mass of $14\,g\,mol^{-1}$.
• Functional Group: An atom or group of atoms in a molecule that determines its specific chemical reactivity and characteristics.
• IUPAC Naming Rules (Carbon Chain Length Prefixes):
  • $1$ carbon atom: Meth-
  • $2$ carbon atoms: Eth-
  • $3$ carbon atoms: Prop-
  • $4$ carbon atoms: But-
  • $5$ carbon atoms: Pent-
  • $6$ carbon atoms: Hex-
• Types of Chemical Formulae:
  • Molecular Formula: Represents the actual number of atoms of each element present in a molecule (e.g., $C_2 H_6$ for ethane).
  • Empirical Formula: Represents the simplest whole-number ratio of atoms of each element present in a compound (e.g., $CH_3$ for ethane).
  • Structural Formula: Shows the spatial arrangement of atoms carbon by carbon, with attached hydrogens and functional groups (e.g., $CH_3 CH_3$ for ethane, $CH_3 CH_2 OH$ for ethanol).
  • Displayed Formula: Shows all atoms and all covalent bonds connecting them in a two-dimensional layout.

Fuels and Fractional Distillation of Petroleum

• Fossil Fuels: Coal, natural gas (composed primarily of methane, $CH_4$), and petroleum (crude oil).
• Petroleum: A complex mixture of hydrocarbons, which are organic compounds containing carbon and hydrogen atoms exclusively.
• Fractional Distillation Process:
  • Petroleum is heated and vaporized in a furnace before being fed into a fractionating column.
  • The column maintains a temperature gradient: hottest at the bottom and coolest at the top.
  • Vapors rise through the column and condense at different heights corresponding to their boiling points.
  • Smaller hydrocarbon molecules have lower boiling points, lower viscosity, and higher flammability; they condense near the top.
  • Larger hydrocarbon molecules have higher boiling points, higher viscosity, and lower flammability; they condense near the bottom.
• Key Petroleum Fractions and Applications:
  • Refinery Gas ($C_1$ to $C_4$): Used as bottled gas for heating and cooking applications.
  • Gasoline / Petrol ($C_5$ to $C_{10}$): Used as fuel for internal combustion engines in automobiles.
  • Naphtha ($C_{10}$ to $C_{16}$): Used as a chemical feedstock for manufacturing plastics and other organic chemicals.
  • Kerosene / Paraffin ($C_{10}$ to $C_{16}$): Used as fuel for jet aircraft engines.
  • Diesel Oil / Gas Oil ($C_{15}$ to $C_{20}$): Used as fuel for diesel engines in trucks, buses, and trains.
  • Fuel Oil ($C_{20}$ to $C_{70}$): Used as fuel for ships and industrial heating systems.
  • Lubricating Oil ($C_{20}$ to $C_{50}$): Used in lubricants, waxes, and machinery polishes.
  • Bitumen ($>C_{70}$): Used for surfacing roads and waterproofing roofs.

Alkanes: Structure, Chemical Reactions, and Substitution

• Definition: Saturated hydrocarbons containing only single covalent bonds between carbon atoms ($C-C$).
• General Formula: $C_n H_{2n+2}$
• Chemical Unreactivity: Alkanes are generally unreactive due to the high bond energy of single $C-C$ and $C-H$ bonds, reacting primarily during combustion and substitution reactions.
• Combustion Reactions:
  • Complete Combustion: Occurs in a plentiful supply of oxygen gas, producing carbon dioxide and water vapor:         $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2 O$
  • Incomplete Combustion: Occurs in a limited supply of oxygen gas, producing toxic carbon monoxide ($CO$) or solid carbon soot ($C$) along with water vapor:         $2CH_4 + 3O_2 \rightarrow 2CO + 4H_2 O$
• Photochemical Substitution Reactions:
  • Alkanes react with halogens (such as chlorine or bromine) in the presence of ultraviolet (UV) light.
  • A hydrogen atom in the alkane is replaced by a halogen atom.
  • Equation for methane and chlorine gas:         $CH_4 + Cl_2 \xrightarrow{\text{UV light}} CH_3 Cl + HCl$

Alkenes: Unsaturation, Cracking, and Addition Reactions

• Definition: Unsaturated hydrocarbons containing at least one carbon-carbon double bond ($C=C$).
• General Formula: $C_n H_{2n}$
• Industrial Production via Catalytic Cracking:
  • Thermal decomposition process that breaks down long-chain alkanes into shorter-chain alkanes and alkenes.
  • Reaction Conditions: Catalyst of alumina ($Al_2 O_3$) or silica ($SiO_2$) at temperatures ranging from $600\,^\circ\text{C}$ to $700\,^\circ\text{C}$.
  • Example cracking equation:         $C_{10} H_{22} \rightarrow C_8 H_{18} + C_2 H_4$
• Chemical Addition Reactions:
  • Test for Unsaturation (Bromination): Addition of aqueous bromine ($Br_2 (aq)$). When shaken with an unsaturated alkene, orange/brown bromine water turns colorless.         $C_2 H_4 + Br_2 \rightarrow C_2 H_4 Br_2$
  • Hydrogenation (Addition of Hydrogen): Converts alkenes into saturated alkanes.
    • Reaction Conditions: Nickel ($Ni$) catalyst at $150\,^\circ\text{C}$.         $C_2 H_4 + H_2 \rightarrow C_2 H_6$
  • Hydration (Addition of Steam): Converts alkenes into alcohols.
    • Reaction Conditions: Concentrated phosphoric(V) acid ($H_3 PO_4$) catalyst, temperature of $300\,^\circ\text{C}$, pressure of $60\,\text{atm}$.         $C_2 H_4 + H_2 O \rightarrow C_2 H_5 OH$

Alcohols: Synthesis, Properties, and Oxidation Reactions

• Functional Group: Hydroxyl group ($\text{-OH}$).
• General Formula: $C_n H_{2n+1} OH$
• Methods of Ethanol Synthesis:
  1. Fermentation of Glucose:
     • Reaction Equation:             $C_6 H_{12} O_6 \xrightarrow{\text{yeast}} 2C_2 H_5 OH + 2CO_2$
     • Conditions: Yeast enzymes, aqueous glucose solution, complete absence of oxygen (anaerobic environment), optimal temperature range of $25\,^\circ\text{C}$ to $35\,^\circ\text{C}$.
     • Evaluation: Uses renewable plant resources; however, it is a batch process yielding dilute ethanol that requires energy-intensive fractional distillation to purify.
  2. Catalytic Hydration of Ethene:
     • Reaction Equation:             $C_2 H_4 (g) + H_2 O (g) \xrightarrow{H_3 PO_4, 300\,^\circ\text{C}, 60\,\text{atm}} C_2 H_5 OH (g)$
     • Evaluation: Fast continuous process yielding pure ethanol; however, it relies on non-renewable crude oil feedstocks.
• Chemical Oxidation of Ethanol:
  • Ethanol is oxidized to ethanoic acid ($CH_3 COOH$) via bacterial oxidation (exposure to atmospheric oxygen) or chemical oxidation using acidified potassium manganate(VII) ($KMnO_4$), which changes color from purple to colorless.
• Combustion of Ethanol:     $C_2 H_5 OH + 3O_2 \rightarrow 2CO_2 + 3H_2 O$

Carboxylic Acids: Weak Acidity, Reactions, and Esterification

• Functional Group: Carboxyl group ($\text{-COOH}$).
• General Formula: $C_{n-1} H_{2n-1} COOH$
• Weak Acidity: Carboxylic acids are weak acids because they only partially dissociate in aqueous solution to release hydrogen ions ($H^+$):     $CH_3 COOH (aq) \rightleftharpoons CH_3 COO^- (aq) + H^+ (aq)$
• Acid Reactions:
  • Reaction with Reactive Metals:         $2CH_3 COOH + Mg \rightarrow (CH_3 COO)_2 Mg + H_2$
  • Reaction with Metal Hydroxides (Bases):         $CH_3 COOH + NaOH \rightarrow CH_3 COONa + H_2 O$
  • Reaction with Metal Carbonates:         $2CH_3 COOH + Na_2 CO_3 \rightarrow 2CH_3 COONa + H_2 O + CO_2$
• Esterification Reaction:
  • Carboxylic acids react with alcohols in the presence of concentrated sulfuric acid ($H_2 SO_4$) catalyst to form an ester and water.
  • Reversible Reaction Equation:         $CH_3 COOH + C_2 H_5 OH \rightleftharpoons CH_3 COOC_2 H_5 + H_2 O$
  • Product Name: Ethyl ethanoate.
  • Properties and Uses: Possess distinctive sweet, fruity smells; used as solvents, food flavorings, and in perfumes.

Polymers: Addition and Condensation Polymerization

• Definition of Polymer: A large macromolecule built up from many smaller repeating units called monomers.
• Addition Polymerization:
  • Occurs when unsaturated monomers containing $C=C$ double bonds join together without forming any side products.
  • Formation of poly(ethene) from ethene monomers:         $n C_2 H_4 \rightarrow -(CH_2-CH_2)_n-$
• Condensation Polymerization:
  • Occurs when monomers join together accompanied by the elimination of a small molecule, such as water ($H_2 O$) or hydrogen chloride ($HCl$).
  • Polyamides (e.g., Nylon):
    • Formed from dicarboxylic acid monomers and diamine monomers.
    • Contain amide linkages ($\text{-CONH-}$).
  • Polyesters (e.g., Terylene / PET):
    • Formed from dicarboxylic acid monomers and diol monomers.
    • Contain ester linkages ($\text{-COO-}$).
• Synthetic vs. Natural Polymers:
  • Synthetic: Poly(ethene), Nylon, Terylene. Non-biodegradable, accumulating in landfill sites and marine environments.
  • Natural: Proteins (amino acid monomers linked by amide bonds) and Carbohydrates (monosaccharide monomers).

Cambridge IGCSE Chapter 19 Organic Chemistry Assessment Quiz

Section A: Multiple Choice Questions

1. Which petroleum fraction is correctly matched with its primary application?

   • A) Naphtha - Fuel for jet aircraft
   • B) Kerosene - Bottled gas for cooking
   • C) Diesel Oil - Fuel for diesel engines
   • D) Bitumen - Chemical feedstock for plastics
   • Answer: C
   • Explanation: Diesel oil is used as fuel for heavy diesel engines. Naphtha is used as a chemical feedstock, Kerosene for jet aircraft, and Bitumen for road surfacing.

2. What are the essential conditions for the direct hydration of ethene to yield ethanol?

   • A) Yeast, $30\,^\circ\text{C}$, anaerobic conditions
   • B) Concentrated $H_3 PO_4$, $300\,^\circ\text{C}$, $60\,\text{atm}$
   • C) Nickel catalyst, $150\,^\circ\text{C}$, $1\,\text{atm}$
   • D) UV light, room temperature, aqueous solution
   • Answer: B
   • Explanation: Direct hydration requires a concentrated phosphoric(V) acid catalyst, a high temperature of $300\,^\circ\text{C}$, and a pressure of $60\,\text{atm}$.

3. An organic compound turns orange bromine water colorless immediately. To which homologous series does it belong?

   • A) Alkanes
   • B) Alcohols
   • C) Carboxylic acids
   • D) Alkenes
   • Answer: D
   • Explanation: Alkenes contain unsaturated carbon-carbon double bonds ($C=C$) that undergo addition reactions with bromine water, decolorizing it.

4. What structural linkage is present in Nylon?

   • A) Ester linkage
   • B) Amide linkage
   • C) Glycosidic linkage
   • D) Carbon-carbon double bond
   • Answer: B
   • Explanation: Nylon is a polyamide formed by condensation polymerization between a dicarboxylic acid and a diamine, creating amide linkages ($\text{-CONH-}$).

5. What product is formed when propanoic acid reacts with ethanol in the presence of concentrated sulfuric acid?

   • A) Ethyl propanoate and water
   • B) Propyl ethanoate and water
   • C) Ethyl propanoate and hydrogen gas
   • D) Ethanoic propanoic anhydride
   • Answer: A
   • Explanation: The ester formed takes its first name from the alcohol (ethyl) and its second name from the carboxylic acid (propanoate), along with water.

Section B: Structured Questions

Question 1: Hydrocarbons and Cracking

Long-chain alkane $C_{12} H_{26}$ is cracked industrially to yield octane ($C_8 H_{18}$) and an alkene monomer X$.\na) State the catalyst and temperature used in catalytic cracking.\n* **Mark Scheme Answer**: Catalyst: Alumina (Al_2 O_3$) or silica ($SiO_2$). Temperature:$600\,^\circ\text{C}$to$700\,^\circ\text{C}.\n\nb) Write a balanced chemical equation for this cracking reaction and identify monomer X$.

• Mark Scheme Answer:     $C_{12} H_{26} \rightarrow C_8 H_{18} + 2C_2 H_4$     Monomer $X$ is ethene ($C_2 H_4$).

c) Describe a chemical test to distinguish between octane and monomer $X$, including observations for both compounds.

• Mark Scheme Answer: Add aqueous bromine water ($Br_2 (aq)$) to separate samples of each compound.
  • Observation with octane (alkane): Solution remains orange/brown.
  • Observation with monomer $X$ (alkene): Solution rapidly turns colorless.

Question 2: Fermentation versus Hydration

Ethanol ($C_2 H_5 OH$) can be manufactured either by fermentation or catalytic hydration. a) Write the balanced chemical equation for the fermentation of glucose ($C_6 H_{12} O_6$).

• Mark Scheme Answer:     $C_6 H_{12} O_6 \xrightarrow{\text{yeast}} 2C_2 H_5 OH + 2CO_2$

b) Give two advantages and two disadvantages of using fermentation rather than catalytic hydration.

• Mark Scheme Answer:
  • Advantages: Uses renewable resources (sugar cane/corn); operates at low temperatures ($25\,^\circ\text{C}$ to $35\,^\circ\text{C}$) saving energy.
  • Disadvantages: Slow batch process; produces dilute ethanol solution requiring energy-intensive fractional distillation.

Question 3: Carboxylic Acids and Esterification

a) Ethanoic acid is a weak acid. Explain what is meant by the term weak acid and write an equation showing its partial dissociation in water.

• Mark Scheme Answer: A weak acid is an acid that only partially ionizes/dissociates in aqueous solution.     $CH_3 COOH (aq) \rightleftharpoons CH_3 COO^- (aq) + H^+ (aq)$

b) Ethanoic acid reacts with ethanol in an esterification reaction. i) State the required catalyst. ii) Write the balanced chemical equation for the reaction. iii) Name the ester product formed.

• Mark Scheme Answer:     i) Catalyst: Concentrated sulfuric acid ($H_2 SO_4$).     ii) Reaction equation:     $CH_3 COOH + C_2 H_5 OH \rightleftharpoons CH_3 COOC_2 H_5 + H_2 O$     iii) Name: Ethyl ethanoate.